Background: Depending on the number of tumor infiltrating lymphocytes, immunological cold to hot conditions vary. There are several clinical trials of administering immune checkpoint inhibitors as perioperative adjuvant therapy. Immune checkpoint inhibitors are generally effective in immunologically hot conditions. However, biopsy specimens are not enough to determine the amount of tumor infiltrating lymphocytes. Therefore, we focused on effector T lymphocytes in peripheral blood, and tried to understand the tumor microenvironment by looking at peripheral blood. Methods: Twenty-four patients with lung cancer who underwent surgery at Fukushima Medical University Hospital from December 2018 to June 2019 were able to separate and collect tumor infiltrating lymphocytes by magnetic cell sorting. Flow cytometry was used to analyze infiltrating lymphocytes and preoperative peripheral blood lymphocytes. Those not expressing CD62L, a marker of Naïve T lymphocytes, were designated as effector T lymphocytes. Results: In the group with a high proportion of cytotoxic T lymphocytes in tumor infiltrating lymphocytes, the proportion of CD62L-negative effector CD4 T lymphocytes in peripheral blood was high (p < 0.05). The percentage of lymphocytes in peripheral blood was also high (p < 0.05). Furthermore, tumor infiltrating lymphocytes had a high proportion of effector CD4 T lymphocytes (p < 0.05). There was a similar trend in the proportion of CD8 T lymphocytes, but there was no statistically significant difference. Conclusions: These results showed that immunologically hot cases could be identified by measuring effector CD4 T lymphocytes in peripheral blood. In the future, we will continue to verify the results and examine antigen specificity of these T lymphocytes.